The proposed studies address an important question regarding the molecular basis of breast cancer: whether the transcription factor TBX3, which has a T-box DNA binding domain, is associated with this condition. Mutations of TBX3 cause Ulnar-Mammary syndrome (UMS) in humans, an autosomal dominant disorder characterized by the absence or underdevelopment of the mammary glands and other congenital anomalies. Abnormal development of the mammary glands in UMS can be partially explained by slow growth of cells in breast tissue due to loss of function of TBX3, indicating that TBX3 is required for normal breast development. Since normal breast development and oncogenesis share several features, it raises the possibility that altered TBX3 gene function may also play a role in the pathogenesis of breast cancer. Our preliminary results have made TBX3 an attractive candidate for breast cancer formation. TBX3 was found to be able to immortalize mouse embryo fibroblast cells (MEF) and TBX3 is overexpressed in many breast cancer cell lines. In addition, TBX2, a homologue of TBX3, was found amplified in some human breast cancers. Here, we propose to study the role of TBX3 in human breast cancer with following specific aims: 1. To test the hypothesis that TBX3 's expression levels and alternative splicing patterns aberrant in 50 human breast cancer tissues with real-time taqMan RT-PCR. Clinical information and pathologic reports will be reviewed and TBX3 expression correlated with other biomarkers and outcomes; 2. To study the inhibition of senescence and oncogenesis activity of TBX3 and TBX3+2a by overexpressing TBX3 in human mammary gland epithelial cells (HMEC). 3. To create a transgenic mouse model by overexpressing TBX3 in the mammary glands to study the role of TBX3 oncogenic activity. There is a high probability that the experiments proposed here will open a new avenue to understanding the pathogenesis of one of the leading causes of death in women, breast cancer. It is our expectation that TBX3 and TBX3+2a will become a breast cancer biomarker and would have a significant application for breast cancer diagnosis and treatment. The animal model created in this application could provide a new paradigm for breast cancer study. By working with both an animal model and tissue acquired from breast surgeries, the proposed research aims to optimize the clinical relevance of the work.